Manufacture of lamp-filaments.



r J. T. MARSHALL. J Y MANUFACTURE OF LAMP FILAM E NTS.

APPLICATION FILED MAY 22, 1968.

996,420, Patented June 27, 1 911.

JOHN T. MARSHALL,

OF METUCHEN, NEW JERSEY, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

MANUFACTURE OF LA MP-FILAMENTS.

Specification of Letters Patent.

Patented June 27, 1911.

Application filed Kay 22, 1908. Serial No. 484,215.

To all whom it may concern:

Be it known that I, JOHN T. MARSHALL, a citizen of the United States, residing at Metuchen, in the county of Middlesex State of New Jersey, have invented certain new and useful Improvements in the Manufacture of Lamp-Filaments, of which the following is a specification.

My present invention consists in an improvement in the manufacture of lamp filaments of that type commonly known to the trade as metallized filaments.

More specifically, my invention consists in so modifying the method of manufacture that the resulting filaments are more dense, uniform, and homogeneous in structure and are more durable and efficient in operation.

The drawing illustrates diagrammatically apparatus that may conveniently be made use of in the practice of my invention commercially. 1

My improved process contemplates the introduction of an additional step into the process of manufacturing the so-called metallized filaments, as previously practiced,described in United States Patent No. 916,905, granted March 30, 1909 to Willis R. Whitney. It will be recalled by those skilled in the art that in the process referred to a base filament of carbon (usually obtained by carbonization of a squirted filament of cellulose out of contact with the air) is fired to a very high temperature to remove impurities, given a carbonaceous coating by the ordinary flashing or treating process of heating with current in benzin vapor, and fired to about the same temperature as before or to a higher temperature. This second heat treat-- ment produces profound changes in the skin or coating and yields the well-known high efficiency filament commonly designated as a metallized filament. This filament is composite, that is to say, it is a filament consisting really of two conductors in parallel, a carbon base, and a shell or skin. The temperature resistance curve of the shell or skin is a curve characteristic of a metal conductor and very different from the curve of ordinary carbon. Without speculating as to the nature of the changes which have gone on during this second firing, it is-suflicient for the present purpose to note that great changes have taken place and the characteristics of the shell have been greatly ting modified. Now it happens that this second firing treatment, whereby the coating of graphite is metamorphosed into a different material, imposes severe strains on the composite filament and tends to break up the union between core and shell and to destroy the homogeneous character of the conductor. It seems very possible that when the changes take place in the shell violent expansion and contract-ion may occur and even complete molecular rearrangements, whereby the shell tends to separate from the core. At all events, I'have found that in some cases it is possible to strip off sections of the shell or coating as little tubes. Furthermore, I have noticed that some filaments as they come from this second firing, have the outer shell projecting slightly beyond the inner core at the end of the filament, much as if the shell had expanded more than the core and slipped out over the end. It is, of course, possible that this peculiar efiect is produced by shrinkage of the core, but at all events it represents a relative movement between the two portions of the filament and indicates the probability that they have been torn apart by the violence of the high heat treatment.

Having described the characteristic features of the process by which metallized carbon filaments have heretofore been commercially manufactured, I will now proceed to indicate how this process can be improved, pointing out some of the advantages accruing from the improvement.

Briefly stated, the improvement consists in so looking the shell to the inner core that relative movement between the two is impossible, and stripping or flaking of the shell does not occur. This resultis' effected by introducing into the process as above set forth, an additional step having for its object the roughening of the core or base prior to its treatment in the benzin vapor, or in other words, prior to the deposit-ion on it of the material which subsequently becomes the metallized shell of the filament.

The rouhening treatment. above mentioned is e ected by heatin the core in the air for a brief time interva This produces oxidation at the surface with resultant pit and roughening. In practice I find it convenient to effect this oxidation or flashing in air by first mounting the filament f on a treating cork C in the usual manner and then touching the terminals t, t against two contacts T, T across which a suitable potential is applied by some sort of an automatic timing device, such for instance as a spur 8 carried on a revolving shaft S, and at each revolution wiping across a spring K acting as a contact. The time of contact of the spur and spring should be a fraction of a second and may be made adjustable by making the spring either nearer to or farther from the spur. In general, the time of treatments should be made long enough to increase the resistance of the filament about 25%. I have found that the voltage should be about three times the voltage which would be necessary to bring the filament to 3.1 watts per candle in a vacuum. This step of flashing in air is the only change contemplated by me in the process of manufacturing metallized filaments. The treatment requires only a moment and yet the benefits are far reaching. For when the coating is put on the core by heatin with current in benzin vapor, the deposited material fills up the pits of the roughened core and takes such a grip on the core that it never Works loose even under the action of those most powerful stresses set up by the very high temperature of the second firing.

From a commercial standpoint the value of my improvement resides principally in an increased life for the lamps, both as regards early breakage and average life. In some instances the improvement has amounted to as much as 50% of the life of metallized filaments as heretofore made.

What I claim as new by Letters Patent of the United States, is

1. As a step in the production of metallized carbon filaments, roughening the base before flashing in the volatile l] \'(.ll.'0(:211l)01'1.

2. As a step in the prodiu-tion of metallized carbon filaments, oxidizing the surface of the base subsequent to the first firing.

3. As a step in the production of metallized carbon filaments, oxidizing the tired base prior to flashing, by heating with an elcctrlc current in air.

4. As a step in the production of metallized carbon filaments, heating the base in air to roughen the surface and prevent subsequent slipping of the superiinposml shell.

5. As a step in the production of metallized carbon filaments. subjecting the base to the action of a suitable reagent to roughen its surface prior to coating the same.

6. As a step in the production of metal lized carbon filaments, subjecting the base to the action .of an oxidizing agent prior to coating the same.

7. As a step in the production of a metallized carbon filament, roughening the base prior to coating and n'1ei'alliz:un.

8. As a step 1n the productioi of. a metallized carbon filament, roughening a fired base prior to coating and lnetallixation.

In witness whereof, I have hereunto set my hand this 19th day of May, 1908.

JOHN T. MARSHALL.

and desire to secure Witnesses S. N. WHITEHEAI), J. HARRY ELnirxs. 

